How to calculate the sampling frequency for a QDa MS method - WKB85513

OBJECTIVE or GOAL

Determine the sampling frequency for an ACQUITY QDa Mass Detector.

ENVIRONMENT

• MassLynx
• ACQUITY QDa Mass Detector

PROCEDURE

1. The instrument cycles through functions sequentially—in order to meet a requested frequency of X, it must acquire a scan from each function X times per second.

   Software works out how much time is available to do this by calculating the cycle time:

   Cycle Time = 1/Frequency

   For example, for a requested frequency of 10 Hz, the cycle time is 0.1 seconds. In this case, the software has 100 ms to divide among the functions.

   On a QDa, the interpolarity delay and interscan delay are not user-definable. The interpolarity delay is 25 ms. The interscan delay is 5 ms (with a lookup table applied to increase this to 6 ms if the mass switch is too large).

2. For same-polarity SIRs overlapping for the entire experiment, the dwell times for each function are calculated as:

   Dwell Time = (Cycle time – delays)/number of functions

   A Sample Frequency of 10 Hz gives:  [100ms – (5 x 5ms)] / 5 = 15 ms dwell time per function

   This would result in a sampling frequency of 10 Hz across the entire experiment.

3. For fewer functions, the dwell time would be increased. 
4. For an MS method with multiple functions at different time points and acquisition times, MassLynx determines the largest number of overlapping functions and uses this to determine the sampling frequency. For example, 25 overlapping functions = 1 / (25 x (5 + 5) /1000) = 4Hz
   If there are fewer overlapping scans (e.g., 20 scans), then you would achieve 5 Hz frequency.
5. For multiple overlapping scans with differing time windows (so in places there are not so many overlapping scans):
   Scans cannot change dwell time halfway through an analysis. So even though, at some points, there are fewer overlapping scans, the overlapping scans will always be the calculated frequency at maximum overlap. The other scans may be higher, but the lower value is always quoted.

ADDITIONAL INFORMATION

For an ACQUITY QDa Mass Detector:

Minimum Dwell Time is 5 ms per scan.
Interscan delay is 5 ms per scan (with a lookup table applied to increase this to 6 ms if the mass switch is too large).